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THE ORGANISM AND ITS ENVIRONMENT

Introduction:

• organisms are not randomly distributed

• there are no homogeneous mixture of organism

• there are restricted patterns of species distribution

• restricted and patchy distributions of organism

• correlations between the biology of different species and the nature of the environment in the ‘patches’ in which they are found

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Ecosystem

Community

Population

Species

Ecosystem ecology (the biotic community & the physical environment in an area)

Community ecology(the study of biotic

communities)

Population ecology (the study of populations of organisms)

Autecology(the study of the individual organism)

The levels of organization in ecology and their associated disciplines

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• abiotic relationships are the primary factors determining whether or not any organism can exist in a certain environment

• physiological processes proceed at different rates under different conditions

• any one organisms only has a limited range of conditions in which it may survive

• tolerance limits: restricts markedly the conditions under which an animal or plant can operate at maximum efficiency

• tolerance curves: curves of performance for any organism for any particular physiological process representing its efficiency of operation over a range of some physicochemical parameter

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a. Evolution by natural selection to explain the match between organisms and environment – Charles Darwin’s (1859) theory of evolution:

1. The individuals that make up a population of a species are not identical

2. This variation is heritable - the characteristics of individuals are determined by its genetic make-up

3. Many individuals in a population reproduce at a less than maximal rate

4. Different individuals leave different number of descendants

5. The number of descendants that an individual leaves depends on the interaction between the characteristics of the individual and the environment of the individual

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Natural selection is the basic mechanism of evolution.

All living organisms are adapted to a specific set of environmental conditions within specific ecosystems

Within every species, however, genetic variation leads to a degree of variation in physical characteristics between different individuals

Some variations allow those who possess them to function more effectively in their particular environment, giving them a greater chance of living long enough and being healthy enough to produce offspring, to which their genes are passed on

natural selection; genetic variations that improve the adaptation of an organism to its environment have a better chance of being passed on than variations that hinder adaptation, simply because better adapted organisms are more likely to survive and reproduce.

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b. Historical Factors:

i. Movements of land masses

• The movement of the tectonic plates of the Earth’s crust, with consequent migration of continents

• Populations were being split and separated, and land areas were moving across climatic zones

ii. Climatic changes

• Changes in climate have occurred on shorter time-scales than the movements of land masses

• The present distribution of species represents phases in a recovery from past climatic shifts

• The present distribution of organisms represents a precise local specialization to present conditions

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iii. Island patterns

• the fauna & flora of islands have several features that distinguish them from the fauna & flora of continents

• there are fewer species on islands than in comparable areas of mainland of the same size

• many of the species on islands are either subtly or profoundly different from those on the nearest comparable area of mainland

• islands contain many species unique to themselves, & many differentiated ‘races’ or ‘sub-species’ that are distinguishable from mainland forms

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PRINCIPLES PERTAINING TO LIMITING FACTORS

1. Liebig’s law of the minimum

• An organism must have essential materials for growth & reproduction

• These basic requirements vary with the species & with the situation

• The essential material available in amounts most closely approaching the critical minimum needed will tend to be limiting one

• ‘growth of a plant is dependent on the amount of food-stuff which is presented to it in minimum quantity’

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2. Shelford’s law of tolerance

• the presence & success of an organism depend upon the completeness of a complex of conditions

• organisms have an ecological minimum & maximum, between which represents the limits of tolerance

• the concept of the limiting effect of maximum as well as minimum – the limits within which various plants & animals can exist

• Some subsidiary principles to the law of tolerance:

i. Organisms may have a wide range of tolerance for one factor & a narrow range for another

ii. Organisms with wide ranges of tolerance for all factors are likely to be most widely distributed

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iii. When conditions are not optimum for a species with respect to one ecological factor, the limits of tolerance may be reduced with respect to other ecological factors

iv. The period of reproduction is usually a critical period when environmental factors are limiting. The limits of tolerance for reproductive individuals, seeds, eggs, etc are usually narrower than for non-reproducing adult plants or animals

• the relative degree of tolerance (steno- (narrow), and eury- (wide)):

i. Stenothermal – eurythermal: temperature

ii. Stenohydric – euryhydric: water

iii.Stenohaline – euryhaline: salinity

iv.Stenophagic – euryphagic: food

v. Stenoecious – euryecious: habitat selection

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TEMPERATURE

AC

TIV

ITY

(G

RO

WT

H)

MIN MINMAX. MAX.

EURYTHERMAL

OPT.

STENOTHERMAL STENOTHERMAL

(OLIGOTHERMAL)OPT.

(POLYTHERMAL)OPT.

Comparison of the relative limits of tolerance of stenothermal & eurythermal organisms

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Physical Factors of Importance as Limiting Factors:

1. Temperature (T)

• T rhythms control the seasonal & daily activities of plants & animals

• T is responsible for the zonation & stratification in waters & land

• Most species & activities are restricted to an even narrower band of temperatures

• The range of T variation tends to be less in water than on land

• Aquatic organisms have a narrower limit of tolerance to T than land animals

• Organisms which are subjected to variable temperatures in nature tend to be depressed, inhibited or slowed down by constant T

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2. Light

• quality of light (wave length or color); the intensity (actual energy); the duration (length of day) are important

• daily rhythms of light & darkness control the movement of animals

• photoperiodicity: the length of day

• reproductive cycle of plants can be classified based on photoperiodicity:

i. long-day plants: bloom when the light duration is > 12h/day

ii. Short-day plants: bloom < 12h duration of light/day

iii.Plants which have wide tolerance to day length, day length is not the limiting factor

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3. Water

• the biotic situation is determined by rainfall & the balance between rainfall and potential evapo-transpiration

• humidity: the amount of water vapor in the air• absolute humidity: the actual amount of water in the air expressed as weight of water per unit of air• relative humidity: represents the percentage of vapor actually present compared with saturation under existing temperature-pressure conditions

• daily rhythm in humidity & vertical & horizontal differences – regulate the activities of organisms

• important role of humidity in modifying the effects of temperature

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4. Atmospheric gases and biogenic salts

• gases are limiting factors in deep soils and mountains

• in aquatic environment, oxygen is limiting in organically polluted waters

• T and dissolved salts affect the ability of water to hold oxygen

• the solubility of oxygen increased by low T and decreased by high salinities

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Limit to performance

Optimum point

Value of controlling environmental variable

Eff

icie

ncy

of

som

e p

hys

iolo

gic

al p

roce

ss

Tolerance curve (Shelford, 1913):• curve of performance for any organism for any particular physiological process

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Physiological optimum

Critical minimum

Critical maximum

Preferred range

Critical limits

Lethal limits

Lower lethal limit

Upper lethal limitE

ffic

ien

cy o

f so

me

ph

ysio

log

ica

l p

roce

ss

Nesting limits to tolerance

• at the lower & upper lethal limits, death occurs• outside the critical max & min, the organisms are ecologically inviable

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Similar curve represents the number of individuals within the population which may found occupying environments at a given value of the chosen physicochemical factor (Shelford, 1913):

• central zone of the normal distribution curve where the majority of the individuals occur: the optimum or preferred range as a whole

• the stress zone: two tails of the curves occupied by relatively few individuals operating under less than optimum conditions

• the tolerance curve can be shifted to a certain extent by genetic or evolutionary changes, or by physiological or behavioural changes during the organism’s lifetime

24• Habitats are actual places which provide numerous niches

The Ecological Niche• the concept which depends on the responses of organisms to environmental conditions

• any given species can survive, grow, reproduce & maintain a viable population within certain environmental limits

• a species’ true ecological niche is an n-dimensional hypervolume within which it can maintain a viable population

• a species can maintain a viable population in a larger ecological niche in the absence of enemies (predators & competitors)

• a niche is an abstract concept that brings together all of an organism’s requirement (environmental conditions) necessary for an organism to maintain a viable population

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The match of organisms to varying environment

3 major categories of environmental change:

i. Cyclic changes: Rhythmically repetitive, e.g. cycles of the

seasons, the movements of the tides, the light & dark periods within a day

ii. Directional changes: The direction of a change is maintained over

a period that may be long in relation to the life span of the organisms, eg. the progressive erosion of a coastline, the progressive deposition of silt in an estuary, cycles of glaciation

iii. Erratic change:All environmental changes that have no

rhythm & no consistent direction, e.g the variation in the time of arrival of monsoon rains, the erratic course & timing of

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hurricanes, cyclones, flash storms, fires caused by lightning

2 main ways in which organisms time their responses to variations in their environment:

i. By changing in response to the environmental change:

If the cycle in conditions is weak & contains much variations, the organisms may best match the changing conditions by responding to them directly, e.g the desert plants germinate in direct response to the arrival of rain

ii. By using a cue that anticipates the change:

Organisms live in environments with strong & repeated cycle of environmental change & the variation in the cycle is relatively weak, e.g the darkness of night & the cold of winter

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Explanations for the diversity that exists within communities:

i. Environments are patchy & heterogenous (made up of different specialized environments)

ii. Most environments contain within them gradients of conditions or of available resources. Some of the diversity species in a community may represent specialization to different conditions along such gradients

iii. The existence of one type of organism in an area immediately diversifies it for others

• Coexisting species commonly differ in subtle ways, but each matches its environment

• The differences in the ecology of species that coexist within communities are most easily recognized when the species are closely related

• Similarities between species may have arisen through convergent evolution or because they have ancestors in common